these sites typically are at medium to low elevations (≤3300 masl) and are believed to have been
short-term, seasonal occupations monitored from
lower-elevation base camps ( 7, 10).

Current archaeological models that address
the timing of a permanent occupation on the
high-elevation step of the plateau postulate that
it could only have been facilitated by the advent
of an agropastoral economy, perhaps as early as
~ 5.2 thousand years cal. B.P., and more certainly,
with the establishment of permanent villages fully
reliant on agriculture, by 3. 6 thousand years
cal. B.P. (1, 4, 5). The presence of securely dated
sites older than 5.2 thousand years from the interior, high-elevation step of the plateau would
challenge these models and would be consistent
with research on the genetics of modern Tibetan
Plateau peoples, which suggests the presence
of a permanent population on the high central
plateau dating to at least 8.0 to 8. 4 thousand years
ago, and possibly as early as the Late Paleolithic
( 11–15).

Here we report the results of an extensive reanalysis of the geochronology, paleoenvironment,
and archaeology of the Chusang site, which is
located on the central plateau 80 km northwest
of Lhasa at an elevation of ~4270 masl, near
Chusang, a village known for its hydrothermal
springs and extensive travertine formations (spring
carbonates deposited by hydrothermal waters)
(Fig. 1). The site, discovered in 1998, consists of
19 human hand- and footprints found on the
surface of a fossil travertine that formed when
hot spring discharge was much higher than it is
today ( 16–18) (supplementary text and fig. S1).
Size variation in the prints suggests that up to
six individuals, including possibly two children,
created them. Optically stimulated luminescence (OSL) dating of quartz grains extracted
from the travertine has indicated that the imprints were created 20 thousand years ago,
making Chusang the only archaeological site
from the interior of the plateau for which a
Paleolithic age assignment is based on a chronometric date ( 16). However, the complex sedimen-tological and dosimetric setting at Chusang raises
the possibility that the luminescence chronology
is severely flawed ( 7, 8).

Although there are other archaeological sites
on the interior of the plateau of reputed Paleolithic age, all are stone tool assemblages of surface finds ( 7, 10), and none has been directly
chronometrically dated. The lack of securely dated
early sites on the central plateau makes a reanalysis of the chronology of Chusang relevant to
a reassessment of current models of the peopling
of the Tibetan Plateau (Fig. 1).

The hand- and footprints are scattered over anarea 20 by 30 m and occur at the surface of asingle sheet of travertine ( 16) (fig. S1). This sheetis up to 1 m thick, extends for several tens ofmeters further to the east, and is underlain bycolluvium (Fig. 2 and figs. S2 and S3). The im-prints are between 2 and 7 mm deep, and theanatomical details of human hands and feet arewell preserved (Fig. 3). The imprints were not en-graved into the travertine, given that evidencefor pecking, scratching, or carving is absent, butmust have formed at a very early stage of hydro-thermal carbonate formation (probably withina few months after calcite precipitation), whenthe top layer of the travertine was still soft anddeformable under the weight of a human body.This mechanism is supported by petrographicevidence showing that the annual travertine layersdirectly below these imprints are continuousbut bended ( 17) (Fig. 3C and supplementarytext). A chronometric age for the travertine thatcarries the imprints will therefore constrainthe time of human presence at these hydro-thermal springs.A thorough understanding of the sedimentol-ogy and petrology of the Chusang travertine, intandem with the application of multiple datingtechniques, is key to developing an accuratechronology for the travertine and the embed-ded hand- and footprints and also importantfor explaining the formation and preservationof the prints ( 18). The travertine is a fossil springdeposit with a total thickness of 24 m. The up-per 11 m of the section are characterized byporous, detrital-rich travertine sheets that areup to 2 m thick and laterally extensive (Fig. 2and and fig. S2). Numerous layers of colluvium[debris flows that originated from the adjacenthill slopes; labeled as diamict massive stratified(Dms) 1 to 4 in Fig. 2 and fig. S2] are intercalatedinto this upper travertine section. The colluviallayers are typically 0.5 to 2 m thick and cancontain (mostly microscopic) organic material.Sedimentology and thin-section microscopy (sup-plementary text and figs. S4 and S5) show thatthe travertine sheets (including the imprintedtravertine) (i) are annually layered on the cen-timeter scale (porous summer and dense winterlayers), (ii) often have clastic microfabrics (e.g.,clasts of redeposited travertine), (iii) have micro-fabrics that are typical for hydrothermal springcarbonates (dendritic crystals), and (iv) precipi-tated synchronously with clastic sediment input(debris flows) from the hill slopes. Many of thefabrics show evidence for recrystallization. Incontrast to this detrital-rich travertine-colluviumsuccession, relatively dense and clean cements areencountered in the pore spaces of the imprintedtravertine (Fig. 3C).A total of 11 samples (nine travertine and twocolluvium) were collected from the upper trav-ertine section for 230Th/U, OSL, and radiocarbondating, as well as for palynological analysis (fig.S1). Of the nine travertine samples, three wereretrieved directly adjacent to imprints, two fromthe same stratigraphic horizon as the imprints,and the remaining four samples from deeperstratigraphic levels. The two colluvium sampleswere collected from stratigraphic positions di-rectly below and 8.2 m below the imprintedtravertine sheet (fig. S1 shows the sampling loca-tions). The details of each dating method arediscussed in the supplementary materials andmethods ( 19) and summarized into one chronol-ogy (Fig. 2).

Travertine is in principle amenable to 230Th/U
dating, but the high detrital content and diagenetic alteration (recrystallization) of the Chusang
travertine impede routine dating. Hence, we followed a modified dating protocol and mapped and
subsampled individual (preferentially primary
and dense) microfabrics to minimize the risk of
sampling altered, potentially recrystallized carbonate ( 19). We used an isochron approach to
account for considerable and variable detrital
contamination. Multiple subsamples can also
help to identify potential problems arising from
open system behavior. Where thick enough, clean
and dense pore cements were targeted for 230 Th/U
dating as well. The three 230 Th/U isochron ages